Nitrogen containing tool steels



3,012,679 NHRJGEN CUNTAINEIG T901. STEELS Reinhold Schempp, Clay, and .ioseph B. Schrader, Baldwinsville, N.Y., assignors to Crucible Steel Company of America, Pittsburgh, Pa, a corporation of New Jersey No Drawing. Filed Feb. 24, 1960, Ser. No. 10,572

12 Claims. (Cl. 75-125) The present invention relates to improved alloy tool steels capable of being hardened by heat treatment. More particularly the invention is drawn to alloy tool steels which are improved by the addition of nitrogen thereto and which are admirably suited for the production of cutting tools.

A primary object of the present invention is to provide improved high speed tool steels containing nitrogen which are capable of being hardened by heat treatment.

Another object of the invention is to provide improved high speed tool steels containing nitrogen which exhibit increased hardness after tempering.

A further object of the invention is to provide improved cutting tools of the aforesaid high speed tool steels which emibit marked cutting properties.

Still another object of the invention is to provide a method of hardening alloy tool steels by the addition of nitrogen thereto in combination with a novel heat treatment.

Further objects of the invention will be obvious to those skilled in the art from the description as hereinafter contained.

The inventive family of improved alloy tool steels includes as necessary adjuvants carbon, manganese, silicon, chromium, vanadium, molybdenum and nitrogen, predicated upon a base of iron. Varying amounts of tungsten and cobalt may be present as impurities or intentionally added for the purpose of varying physical properties other than those in which the point of novelty resides. Commercial impurities such as phosphorus, sulphur and nickel and residual deoxidizers, such as aluminum, may be present in minor amounts without afiecting the novel characteristics of the invention.

The improved alloy tool steels of the invention are typified by the following analyses:

A preferred range of analyses within the foregoing limits is as follows:

Carbon 0.85 to 0.95%. Manganese 0.25 to 0.50%.

Silicon 0.20 to 0.50%. Chromium 3.50 to 4.50%. Vanadium 1.50 to 2.50%. Tungsten Up to 0.75%. Molybdenum 7.50 to 8.50%.

Cobalt Up to 0.5%.

Nitrogen 0.08 to less than 0.13%.

Iron plus impurities Balance.

3,012,879 Patented Dec. 12, 1961 The following tabulation is presented as illustrative of alloys prepared during the perfection of this invention:

Heats A, B and C, in thirty pound lots, were cast into four inch square ingots, forged to bars of one inch square cross-section and fully annealed.

Samples one inch by one inch by one-half inch were cut from said bars and treated in a molten salt bath. The samples were put into baskets With a dummy load and after a preheat at 1550 F. were hardened by quenching from temperatures of 2150 F., 2175 F., 2200 F, 2225 F. and 2250 F., respectively. Three samples were hardened from each temperature by holding at temperature for one minute and quenching in oil.

After hardening, one of each set of three samples was tempered at 975 F., 1025 F. and 1075 F. for 2 plus 2 hours. Some of the samples were tempered an extra 2 hours as indicated where examination of the microstructure revealed incomplete tempering after 2 plus 2 hours.

TABLE II Response to heat treatment of steel A Rockwell G as tempered Rockwell Hardening temperature, F. C as quenched 975 F., 1,025 R, 1,075 E, 2+2 hrs. 2+2 hrs. 2+2 hrs.

TABLE III Response to heat treatment of steel B 1,075 Ft, 2+2 hrs.

Hardening temperature, F.

quenched 975 F.,

1,025 F., 2+2 hrs.

2+2 hrs.

3 TABLE IV Response to heat treatment of steel C It will be noted from Table II that the temper hardness of steel A is substantially the same as or less than the as quenched hardness in the majority of cases tested and effectively increased in but two such cases. Since all but the latter hardness values are about normal for alloy tool steels containing no nitrogen but otherwise having compositions falling within the broad ranges of the invention, it is apparent that the presence of 0.12% nitrogen in steel A merely compensated for the absence of carbon, the contained carbon being 0.07% below the minimum carbon content of the invention.

Steel B test results, as shown in Table III, indicate little increase in temper hardness over steel A at a temper temperature of 975 F., but substantial increases on the order of two Rockwell C hardness units at temper temperatures of 1025 F. and 1075 F.

Temper hardness values for steel C, given in Table IV, are even more impressive than steel B values in that increases are obtained for all three temper temperatures tested, namely, 975 F., 1025 F. and 1075 F.

The low as quenched hardness values of steels B and C indicate the austenite forming tendency of nitrogen. This is borne out by examination of the microstructure of the as quenched samples of said steels wherein abnormal amounts of retained austenite are observed, that is, amounts of retained austenite substantially greater than that which is characteristic of steels containing substantially no nitrogen but otherwise having compositions falling within the broad ranges of the invention. On the other hand, the relatively high as quenched hardness values of steel A indicate the predominant presence of martensite and, therefore, the criticality of the carbon content, since the retention of austenite is not increased by nitrogen where said carbon content is without the critical range of 0.85 to 0.95%.

It can be theorized that since steels B and C have more retained austenite than steel A, tempering of steels B and C should result in greater hardness increases per so, that is, a greater number of hardness units added to the as quenched hardness in attaining temper hardness, since steels B and C contain more austenite which can undergo transformation to relatively hard martensite. Examination of Tables II, III and IV proves this to be the case. However, that the ultimate temper hardness of steels B and C will substantially exceed that of steel A cannot so easily be theorized, and the unobviousness of this result is indicative of the high order of the present invention. It is hypothesized that the transformation of retained austenite to martensite is but part of the hardening mechanism and that the critical proportioning of essentially nitrogen, carbon, vanadium and chromium results in precipitation of hard intermetallic particles of vanadium and chromium nitrides and carbides. It is important to note that despite the increased hardness upon tempering of steels of the present invention there is no apparent impairment of the workability of such steels in the hot or cold state.

In order to determine the relative cutting properties of cutting tools made from steels of the present invention, cutters were fashioned from steels of the invention, des

ignated as M-lO/N, and also from steels containing substantially no nitrogen but otherwise having compositions falling within the broad ranges of the invention, designated as M10. The cutters were given similar heat treatments, namely, quenching in oil from a hardening temperature in the range of 2150 F. to 2250 F. followed by tempering in the range of 950 F. to 1100 F. The resultant heat-treated cutters were subjected to a performance test, the results of whch are illustrated below:

Shell End Mill Test- 1 4" die. 8 tooth shell end mills.

' Super HY-Tuf steel50 Rockwell 0".

36 r.p.m., 2 A" feedsultur base oil coolant.

Avg. No. Relative Steel of cuts for Cutting 6 runs Ability,

percent M10/N 24. 5 100 M-IO 9. 2 33 In this test, the steel to be tested is formed into a mill or milling machine cutter comprising a hollow cylinder with teeth on the outside cylindrical surface. One end oi the cylinder is closed cit to form a base to which a. drive shaft is connected. During the test, work is fed back and forth along a planar surface in such manner as to contact the teeth of the rotating cutter, which rotates about an axis thru the center of said heft.

2 Registered trademark-Crucible Steel Company of America.

The above test results leave no doubt of the marked superiority of cutting tools made of the steels of the present invention over cutting tools of similar steels without the addition of nitrogen. Further, the former have been observed to out-perform the latter in cutting tests even where the latter were made of steels of equivalent hardness, the increased abrasion resistance attendant the addition of nitrogen to the former being apparently responsible therefor.

From the foregoing it is obvious that the instant invention provides a method for substantially increasing the hardness of tool steels with predictability and certainty thus enabling the production of cutting tools possessing superior cutting qualities and this in an already overcrowded field where an increase of as little as 0.5 Rockwell C hardness units constitutes a patentable advance in the art.

Having now described several forms of the invention, it is expressly pointed out and understood that said invention is not to be limited to the specific embodiments or arrangements of steps hereinbefore disclosed, except insofar as such limitations are specified in the appended claims.

What is claimed is:

1. An improved alloy tool steel capable of being hardened by heat treatment consisting essentially of 0.85 to 0.95% carbon, 0.25 to 0.50% manganese, 0.20 to 0.50% silicon, 2' to 7% chromium, 0.5 to 5% vanadium, up to 10% tungsten, 1 to 12% molybdenum, 0.05 to less than 0.13% nitrogen, up to 10% cobalt, balance iron plus commercial impurities.

2. An improved alloy tool steel capable of being hardened by heat treatment consisting essentially of 0.85 to 0.95% carbon, 0.25 to 0.50% manganese, 0.20 to 0.50% silicon, 3.50 to 4.50% chromium, 1.50 to 2.50% vanadiurn, up to 0.75% tungsten, 7.50 to 8.50% molybdenum, up to 0.5% cobalt, 0.08 to less than 0.13% nitrogen, balance iron plus commercial impurities.

3. A method of imparting increased attainable hardness in an alloy tool steel consisting essentially of 0.85 to 0.95% carbon, 0.25 to 0.50% manganese, 0.20 to 0.50% silicon, 2 to 7% chromium, 0.5 to 5% vanadium, up to 10% tungsten, 1 to 12% molybdenum, up to 10% cobalt, balance iron plus commercial impurities, comprising the steps of adding nitrogen to a melt of said steel in an amount effective to produce a residue to 0.05 to less than 0.13% therein, casting the nitrogen containing steel into ingots, forging and annealing said ingots, heating the resultant product at a temperature within the range of about 2150 to 2250 F., quenching and re heating at a temperature within the range of about 950 to 1100 F.

4. The method of claim 3 wherein the re-heating temperature is within the range of about 1000 to 1100 F.

5. A method of imparting increased attainable hardness in an alloy tool steel consisting essentially of 0.85 to 0.95% carbon, 0.25 to 0.50% manganese, 0.20 to 0.50% silicon, 3.50 to 4.50% chromium, 1.50 to 2.50% vanadium, up to 0.75 tungsten, 7.50 to 8.50% molybdenum, up to 0.5% cobalt, balance iron plus commercial impurities, comprising the steps of adding nitrogen to a melt of said steel in an amount effective to produce a residue to 0.05 to less than 0.13% therein, casting the nitrogen containing steel into ingots, forging and annealing said ingots, heating the resultant product at a temperature Within the range of about 215'0 to 2250 F., quenching and re-heating at a temperature within the range of about 950 to 1100 F.

6. A tempered cutting tool characterized by superior cutting properties and comprising an alloy tool steel consisting essentially of 0.85 to 0.95% carbon, 0.25 to 0.50% manganese, 0.20 to 0.50% silicon, 2 to 7% chromium, 0.5 to 5% vanadium, up to 10% tungsten, 1 to 12% molybdenum, 0.05 to less than 0.13% nitrogen, up to 10% cobalt, balance iron plus commercial impurities.

7. A tempered cutting tool characterized by superior cutting properties and comprising an alloy tool steel consisting essentially of 0.85 to 0.95% carbon, 0.25 to 0.50% manganese, 0.20 to 0.50% silicon, 3.50 to 4.50% chromium, 1.50 to 2.50% vanadium, up to 0.75% tungsten, 7.50 to 8.50% molybdenum, up to 0.5% .cobalt, 0.08 to less than 0.13% nitrogen, balance iron plus commercial impurities.

8. An improved cutting tool characterized by superior hardness and cutting properties and consisting essentially of 0.85 to 0.95% carbon, 0.25 to 0.50% manganese, 0.20 to 0.50% silicon, 2 to 7% chromium, 0.5 to 5% vanadium, up to 10% tungsten, 1 to 12% molybdenum, 0.05 to less than 0.13% nitrogen, up to 10% cobalt, balance iron plus commercial impurities.

9. An improved cutting tool according to claim 8, wherein said cutting tool is in the condition obtained by heating at a temperature within the range of about 2150 to 2250 F., quenching and re-heating at a temperature within the range of about 950 to 1100 F.

10. An improved cutting tool according to claim 9, wherein said re-heating temperature is within the range of about 1000 to 1100" F.

11. An improved cutting tool characterized by superior hardness and cutting properties and consisting essentially of 0.85 to 0.95% carbon, 0.25 to 0.50% manganese, 0.20 to 0.50% silicon, 3.50 to 4.50% chromium, 1.50 to 2.50% vanadium, up to 0.75% tungsten, 7.50 to 8.50% molybdenum, up to 0.5% cobalt, 0.08 to less than 0.13% nitrogen, balance iron plus commercial impurities.

12. An improved cutting tool according to claim 11, wherein said cutting tool is in the condition obtained by heating at a temperature within the range of about 2150" to 2250 F., quenching and re-heating at a temperature within the range of about 950 to 1100 F.

Australia Aug. 1, 1940 Austria Dec. 10, 1956 

1. AN IMPROVED ALLOY TOOL STEEL CAPABLE OF BEING HARDENED BY HEAT TREATMENT CONSISTING ESSENTIALLY OF 0.85 TO 0.95% CARBON, 0.25 TO 0.05% MANGANESE, 0.20 TO 0.50% SILICON, 2 TO 7% CHROMIUM, 0.5 TO 5% VANADIUM, UP TO 10% TUNGSEN, 1 TO 12% MOLYBDENUM, 0.05 TO LESS THAN 0.13% NITROGEN, UP TO 10% COBALT, BALANCE IRON PLUS COMMERCIAL IMPURTIES.
 3. A METHOD OF IMPARTING INCREASED ATTAINABLE HARDNESS IN ALLOY TOOL STEEL CONSISTING ESSENTIALLY OF 0.85 TO 0.95% CARBON, 0.25 TO 0.50% MANGANESE, 0.20 TO 0.50% SILICON, 2 TO 7% CHROMIUM, 0.5 TO 5% VANADIUM, UP TO 10% TUNGSTEN, 1 TO 12% MOLYBDENUM, UP TO 10% COBALT, BALANCE IRON PLUS COMMERICAL IMPURITIES, COMPRISING STEPS OF ADDING NITROGEN TO A MELT OF SAID STEEL IN AN AMOUNT EFFECTIVE TO PRODUCE A RESIDUE TO 0.05 TO LESS THAN 0.13% THEREIN, CASTING THE NITROGEN CONTAINING STEEL INTO INGOTS, FORGING AND ANNEALING SAID INGOTS, HEATING THE RESULTANT PRODUCT AT A TEMPERATURE WITHIN THE RANGE OF ABOUT 2150* TO 2250*F., QUENCHING AND REHEATING AT A TEMPERATURE WITHIN THE RANGE OF ABOUT 950* TO 1100*F. 